The first floor of the Tower is made up of Darwiniancreatures, named for Charles Darwin's theory of natural selection. These creatures have various genotypes, {W, X, Y, Z}, that code for phenotypes {W, X, Y, Z}. The phenotypes are variations on one biologicalmodel When confronted with situation Q in its environment, each phenotype reacts differently. Phenotypes W, X, and Z tend to react to situation Q in a way that will keep the creature with the corresponding genotype from being able to reproduce, by killing the creature or otherwise ending its "family tree" as a result of situation Q. Only phenotype Y tends to allow survival/reproduction, which means that genotype Y will soon outnumber W, X, and Z in the environment. Darwinian creatures are "hard-wired" and can only react in the narrow ways their phenotype will allow. Individual creatures are unable to choose the options available, and are thus largely insignificant to evolution on this floor.

The next floor is made up of Skinnerian creatures, named for B.F. Skinner and his school of psychology known as behaviorism. These creatures have a phenotype that can offer options W, X, Y, and Z when confronted with situation Q. They are also equipped with an ability to recognize positive and negative input from their environment, but otherwise "blindly" try random options when confronted with situation Q. Options W, X, and Z tend to cause negative input from the environment, while option Y tends to cause positive input. Eventually, the individual creature will tend to use option Y when confronted with situation Q. B.F. Skinner's behaviorism is the theory that all animals, from sea slugs to people, are these types of creatures. Dennett states, "Skinnerian conditioning is a fine capacity to have, so long as you are not killed by one of your early errors". That is, individual creatures can eventually become more fit for their environments, as long as they don't choose option X first, which leads to immediate death or failure to reproduce.

The third floor contains creatures with a system of preselection, a mental ability to weed out the "truly stupid" options before attempting them in the real world - that is, ruling out options X and Z altogether when first encountering situation Q, because the creature's mind can foresee bad consequences without prior conditioning for that exact situation. These creatures are called Popperian, for Sir Karl Popper, who once said that this designelement "allows our hypotheses to die in our stead". This will make the creature more likely to choose option Y immediately, when first facing situation Q, and thus survive and reproduce. Fit Popperian creatures can be differentiated from fit Skinnerian creatures: the former are smart enough to make better-than-chance first moves, while the latter are only lucky enough to make the right first moves.

Popperian creatures, in Dennett's view, make up most of the animals used in behavioral studies: they have an innerenvironment, however primitive, that allows them to preselect options. Dennett notes that the sea slug has replaced the pigeon in Skinnerian studies, as pigeons (Skinner's favorites) and other creatures that are not simple invertebrates can be shown to exhibit some preselection. Even the octopus has been shown to be an astoundingly smart Popperian creature by its ability for preselection. But is there a separation of degree between humans and other Popperian creatures? After all, humans do their share of mindless preselection. But Popperian creatures have limits that go back to their biological origins: that is, if Darwinian natural selection has made options Y and Z "unthinkable" to the creatures because of their geneticnature, then Popperian preselection cannot consider those options. It is on this level that psycholinguist Noam Chomsky places human thought, hypothesizing that there are certain things that humans cannot know, in the same way that fish cannot understand what we call "fishing", much less democracy. Chomsky has theorized that perhaps finding the answer to the problem of free will is such an option, ruled out by our biology and prior selection.

Dennett posits the final level of the tower as that at which humans have arrived: that of science. This is a system that we have created using Gregorian methods of foresight and earlier methods of trial-and-error to embody a better system of generate-and-test that is less arbitrary than all the rest. Situations can be fed into this singular system and accurate results can be gathered without having to actually encounter the situation firsthand - what is known as a hypothesis. Science is also not just a matter of making mistakes, but of making mistakes in public - that is, the human capacity for language allows for a single individual to build upon the results of earlier scientific tests by proposing new experiments for new hypotheses. Thus, there is something of a new genetics in the science of ideas, where old ideas that do not work, like the Ptolemaic theories, do not need to be theorized by each individual and subsequently demolished in order to be replaced by more relevant ideas as the prevailing understanding of a given situation (in this case, the solar system). This makes naked animal Popperian brains no match for our heavily-equipped scientific society, and reverses the burden of proof on Chomsky and other theorists who posit "cognitive closure" (the aforementioned theory that human brains are simply closed to consideration of certain options, even when they are "obvious"). Science provides for a new way of solving problems that can encompass things outside of the Gregorian mien we are born into.

(An interesting project would be a writeup of how this theory applies to Everything2 and its noders.)